1. Field of the Invention
This invention relates generally to intraocular lenses and, more particularly, to intraocular lenses having haptics adapted for scleral fixation.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of a typical, healthy human eye 100. The human eye is a generally spherical body defined by an outer wall called the sclera 110, having a transparent bulbous front portion called the cornea 112. The lens 114 of the human eye is located within the spherical body, behind the cornea. The iris is located between the lens and the cornea, dividing the eye into an anterior chamber 120 in front of the iris and a posterior chamber 118 in back of the iris. A central opening in the iris, called the pupil 122, controls the amount of light that reaches the lens. Light is focused by the cornea and by the lens onto the retina at the rear of the eye. The lens is a bi-convex, highly transparent structure surrounded by a thin lens capsule 126. The lens capsule is supported at its periphery by suspensory ligaments called zonules 128, which are continuous with the ciliary muscle 130. The focal length of the lens is changed by the ciliary muscle. Just in front of the zonules, between the ciliary muscle and iris, is a region referred to as the ciliary sulcus 132.
A cataract condition results when the material of the lens 114 becomes clouded, thereby obstructing the passage of light. To correct this condition, three alternative forms of surgery are used, known as intracapsular extraction, extracapsular extraction, and phacoemulsification. In intracapsular cataract extraction, the zonules 128 around the entire periphery of the lens capsule are severed, and the entire lens structure, including the lens capsule 126, is then removed. In extracapsular cataract extraction and phacoemulsification, only the clouded material within the lens capsule is removed, while the transparent posterior lens capsule wall with its peripheral portion, as well as the zonules, are left in place in the eye.
Intracapsular extraction, extracapsular extraction, and phacoemulsification eliminate the light blockage due to the cataract condition. The light entering the eye, however, is thereafter unfocused due to the lack of a lens. A contact lens can be placed on the exterior surface of the eye, but this approach has the disadvantage that the patient has virtually no useful sight when the contact lens is removed. A preferred alternative is to implant an artificial lens, known as an intraocular lens, directly within the eye. An intraocular lens generally comprises a disk-shaped, transparent lens optic and two smoothly curved attachment arms referred to as haptics. The lens is implanted through an incision made near the periphery of the cornea, which may be the same incision as is used to remove the cataract. An intraocular lens may be implanted in either the anterior chamber of the eye, in front of the iris, or in the posterior chamber, behind the iris.
An anterior chamber lens is supported by contact of the haptics with a corner, or angle, of the anterior chamber 120 which is formed by the union of the iris 116 and the cornea 112. In the case of a posterior chamber lens, there are two alternative techniques of support. In the first technique, the intraocular lens and its haptics are placed in the sack-like structure formed by the intact posterior and peripheral walls of the lens capsule. The haptics are compressed slightly against the periphery of the lens capsule and thereby hold the intraocular lens in place. In the second technique, the intraocular lens is placed in front of and outside the lens capsule. The haptics are sandwiched between the iris 116 and the zonules 128, in the region of the ciliary sulcus 132, to hold the lens in place. Anterior chamber lenses differ significantly in design from posterior chamber lenses. Accordingly, one cannot be used in substitution for the other.
Experience has shown that intraocular lenses implanted in the anterior chamber dramatically increase the risk of significant complications. Anterior chamber lenses have been known to cause hyphema, uveitis, iris chaffing, glaucoma, cystoid macular edema, persistent ocular pain and redness, and pseudophakic bullous keratopathy. As a result of treating these conditions and other complications, anterior chamber lenses need to be explanted, or removed, approximately 20 times more often than posterior chamber lenses. For these reasons, when adequate zonular or capsular support is present, the posterior chamber lens generally remains the surgeon's first choice.
Unfortunately, adequate zonular or capsular support is not-always present. Certain abnormalities of the eye, traumatic injury to the eye, previous intracapsular surgery, or an intra-operative complication during extracapsular or phacoemulsification surgery, all can result in a loss of the capsular or zonular support for a posterior chamber lens. Until recently, the surgeon has had no choice but to implant an anterior chamber lens.
However, surgeons have recently succeeded in implanting intraocular lenses in the posterior chamber in the absence of capsular and zonular support by suturing the lens to the sclera of the eye. The lens haptics are usually sutured to the sclera at or near the ciliary sulcus. The limited number of surgeons who have experimented with scleral fixation have reported promising results. Early reports show that intraocular lenses attached to the eye by scleral fixation are well tolerated, provide good vision, and bear significantly fewer complications than anterior chamber lenses. If these promising results continue to be confirmed, scleral-fixated posterior chamber lenses could make anterior chamber lenses obsolete.
Standing in the way of rapid adoption of scleral fixation for posterior chamber lenses is the fact that specialized lenses and materials to perform the procedure are not yet available. Most ophthalmic surgeons can easily acquire the surgical skills needed for scleral fixation. Suitable sutures and needles are presently available for the procedure, such as the 10-prolene suture and the Ethicon CIF-4 or STC-6 needles. However, intraocular lens designed specifically for scleral fixation are not widely known or available.
At present, ophthalmic surgeons ordinarily use conventional posterior chamber lenses and simply tie the fixation sutures to the lens haptics. There are several drawbacks to this technique. First, because the fixation suture is tied to the smooth haptic, migration of the haptic and lens optic relative to the suture is possible. Such migration could lead to lens decentration, lens tilt, or even complete loss of lens fixation. Second, the process of tying the suture to the lens haptic (generally with a double knot or a clove hitch) is a difficult, time consuming, and often frustrating procedure. If the suture is tied to the haptic with a double knot, the end of the suture may irritate the inside of the eye. These problems are amplified by the need to repeat the procedure for each haptic. Third, current posterior chamber lenses are considered too large for scleral-fixated placement in the typical posterior chamber. The diameter of the typical ciliary sulcus measures about 11 millimeters, while the typical posterior chamber lens measures 14 millimeters across. Finally, the haptics of current posterior chamber lenses do not provide optimal stability of the lens optic in the absence of the lens capsule. Such instability can lead to lens tilting and can introduce uncorrectable optical problems after scleral fixation.
Some ophthalmic surgeons have experimented with sclerally fixating lenses of the kind that include instrument positioning holes at various positions along their haptics. These holes have sizes typically of 0.50 millimeters, but are known to have sizes as low as 0.34 millimeters. The use of this type of lens has not proven to be entirely satisfactory, because the implanted lenses are believed to have been subject to excessive tilting, decentration and rotation. In addition, the instrument positioning holes of such implanted lenses are subject to tissue ingrowth and incarceration, which can lead to lens decentration and can make later removal or repositioning of the lenses difficult or impossible to perform.
It should therefore be appreciated that there is a need for an intraocular lens, and a method for implanting it in the eye by scleral fixation, which is convenient to implant and which is sufficiently stable when implanted that it minimizes the possibility of tilting or decentering. The present invention fulfills this need.